The Collaborative International Dictionary of English v.0.48:

Sisterly \Sis"ter*ly\, a.
   Like a sister; becoming a sister, affectionate; as, sisterly
   kindness; sisterly remorse. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sisterly
    adj 1: like or characteristic of or befitting a sister;
           "sisterly kindness"; "sororal concern" [syn: sisterly,
           sisterlike, sororal] [ant: brotherlike,
           brotherly, fraternal]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "sisterly":
   agreeable, amiable, amicable, brotherly, congenial, favorable,
   fraternal, friendlike, friendly, harmonious, kind, neighborlike,
   neighborly, peaceable, pleasant, simpatico, sociable, sympathique,
   unhostile, well-affected, well-disposed, well-intentioned,
   well-meaning, well-meant